Linking vegetation change, carbon sequestration and biodiversity: insights from island ecosystems in a long‐term natural experiment

Abstract

Summary1. Despite recent interest in linkages between above-and below-ground communities and their consequences for ecosystem processes, much remains unknown about their responses to long-term ecosystem change. We synthesize multiple lines of evidence from a long-term 'natural experiment' to illustrate how ecosystem retrogression (the decline in ecosystem process rates due to long-term absence of major disturbance) drives vegetation change, and thus above-ground and below-ground carbon (C) sequestration, and communities of consumer biota. 2. Our study system involves 30 islands in Swedish boreal forest that form a 5000-year, fire-driven retrogressive chronosequence. Here, retrogression leads to lower plant productivity and slower decomposition and a community shift from plants with traits associated with resource acquisition to those linked with resource conservation. 3. We present consistent evidence that above-ground ecosystem C sequestration declines, while below-ground and total C storage increases linearly for at least 5000 years following fire absence. This increase is driven primarily by changes in vegetation characteristics, impairment of decomposer organisms and absence of humus combustion. 4. Data from contrasting trophic groups show that during retrogression, biomass or abundance of plants and decomposer biota decreases, while that of above-ground invertebrates and birds increases, due to different organisms accessing resources via distinct energy channels. Meanwhile, diversity measures of vascular plants and above-ground (but not below-ground) consumers respond positively to retrogression. 5. We show that taxonomic richness of plants and above-ground consumers are positively correlated with total ecosystem C storage, suggesting that conserving old-growth forests simultaneously maximizes biodiversity and C sequestration. However, we find little observational or experimental evidence that plant diversity is a major driver of ecosystem C storage on the islands relative to other biotic and abiotic factors. 6. Synthesis. Our study reveals that across contrasting islands differing in exposure to a key extrinsic driver (historical disturbance regime and resulting retrogression), there are coordinated responses of soil fertility, vegetation, consumer communities and ecosystem C sequestration, which all feed back to one another. It also highlights the value of well-replicated natural experiments for tackling questions about above-ground-below-ground linkages over temporal and spatial scales that are otherwise unachievable.

How does pedogenesis drive plant diversity?

“…Soil chronosequences, by contrast, enable comparisons between ecosystems within much smaller regions, where factors such as climate and parent material are controlled [50,51]. The available data reveal that total plant species richness usually increases with soil age across soil chronosequences spanning boreal, temperate, subtropical, and Mediterranean climates [4,5,24] (Figure II). Together, these global and smallerscale patterns motivate the search for underlying mechanisms that can explain the greater plant diversity on older soils.…”

“…As a result, peak productivity coincides with a relatively 'balanced' supply of inorganic and organic N, potentially generating a greater opportunity for N partitioning. As retrogression proceeds, more of the total soil N pool occurs in less available organic forms (e.g., protein-tannin complexes), N mineralization rates decrease strongly, and a larger proportion of N is supplied as amino acids [23,24]. Partitioning of organic N would still be possible where the majority of the soluble N supply is in the form of amino acids, given that plant species can show preferences for specific amino acids or peptides [18].…”

Genotypic variability in Populus tremula L. affects how anthropogenic nitrogen enrichment influences litter decomposition

“…Litter decomposition is a key soil process that is potentially influenced by N enrichment, and which is critical for nutrient recycling in boreal forests (Prescott 2010;Wardle et al 2012). Studies that have looked at how decomposition is affected by N, however, have not yielded consistent results.…”

A Review of Strong Evidence for the Effect of Functional Dominance on Carbon Stocks in Natural Forest Ecosystems

“…In some studies, the species or functional groups have been experimentally removed from initially homogeneous woody communities in the field (Aguiar and Sala, 1994;Diaz et al, 2003;Bret-Harte et al, 2008;Wardle et al, 2008;Urcelay et al, 2009). Further, many studies have been independently tested the effects of different components of FTD, particularly CWM and FD effects, in natural forests and also assessed the trait-specific relationships with C stocks in natural forest ecosystems (Caspersen and Pacala, 2001;Delagrange et al, 2008;Jonsson and Wardle, 2010;Ruiz-Jaen and Potvin, 2011;Wardle et al, 2012). However, empirical studies focusing on the comparison of CWM (the mass ratio hypothesis) and FD (the niche complementarity) effects on C stocks in the naturally established forest ecosystems are still very scarce (Conti and Diaz, 2013;Cavanaugh et al, 2014;Finegan et al, 2015).…”